1. Field of the Invention
The present invention relates to a hydraulic machine having a hydro turbine runner, i.e., a Francis turbine, and in particular to a technique to grow in the performance of a hydro turbine by improving the shape of a splitter runner blade.
2. Description of the Related Art
A Francis turbine incorporated in a hydraulic machine has runner blades 102 at regular intervals along the circumference of a rotary runner 101, as shown in FIG. 13. In FIG. 13, reference numbers 103 and 104 denote a crown and a band, respectively, which support the runner blades 102. An arrow F indicates a flow of water, and a chain line C indicates the axis of rotation.
In the above hydraulic machine, the volume of water flowing into the runner 101 is adjusted by changing the opening of a guide vane (not shown), and the amount of generated electrical energy is changed. Thus, a flow in the runner 101 is greatly changed by the volume of water, and the water flow F from a guide vane forms a streamline along the runner 101 at a design point.
However, as the water discharge decreased, the flow in the runner is deviated to the band 104 by the centrifugal force by the rotation of the runner. In contrast, when the discharge increased, the energy from the guide vane to the center of the runner 101 increased to larger than the centrifugal force by the rotation of the runner 101, and the flow in the runner is deviated to the crown 103. This flow not along a streamline is called a secondary flow. A secondary flow causes a loss in the runner 101. If a secondary flow can be prevented in either a large-flow part or a small-flow part, it is possible to provide the runner 101 with low power loss in a wide range.
As a known means of preventing a secondary flow, there is a splitter runner 110 having short blades 112 between ordinary long blades 111, as shown in FIG. 14. An arrow R in the drawing indicates a rotation direction of the runner in turbine operation. It is known that the short blades 112 of the split runner 110 prevent deviation of flow at a design point in particular, and decrease power loss. This prevents deviation of flow to the band in part-load operation, as shown in FIG. 15.
In the above splitter runner 110, rear edges 111a of the long blades 111 and rear edges 112a of the short blades 112 are made linear from the band 104 to the crown 103, and arranged radial to the axis of rotation C, as shown in FIG. 16.
As a technique concerning a splitter runner, a technique concerning the shape of a rear edge of a short blade is known in particular. For example, a front edge of a short blade is retracted from a crown toward a band with respect to a rotation direction in pump operation, and the cavitation performance in pump operation is increased. (Refer to Published Japanese Patent No. 3782752.) A rear edge of a short blade is retracted from a band toward a crown with respect to a rotation direction in turbine operation, and the cavitation performance in turbine operation or pump operation is increased. (Refer to Jpn. Pat. Appln. KOKAI Publication No. 2003-65198.)
A hydraulic machine having the above splitter runner has the following problems. A secondary flow can be prevented in large and small water discharge, but the efficiency of a hydro turbine cannot be increased in a wide range of water discharge.